In computer science, all number values have a range limit
and if you go beyond that, the number wraps around to zero. In this case, you are specifying a value that goes beyond the range of what mIRC can handle.
When /timer was first added, I should have placed a reasonable maximum, say one day or one week, on the timer interval. I could do this now, however it would cause all existing scripts that go beyond this value to trigger incorrectly.
The maximum /timer interval value is not easy to determine because the value is used in a number of computations, so the actual range will be less than the maximum range for a signed long.